1. Field of the Invention
The present invention relates to an optical system having a reflecting surface of non-rotationally symmetric surface configuration. More particularly, the present invention relates to an optical system suitable for use in a head- or face-mounted image display apparatus that can be retained on the observer's head or face.
2. Discussion of Related Art
As an example of a conventional head- or face-mounted image display apparatus, an image display apparatus disclosed in Japanese Patent Application Unexamined Publication (KOKAI) No. 3-101709 (1991) is known. FIG. 19(a) shows the entire optical system of the conventional image display apparatus, and FIG. 19(b) shows a part of an ocular optical system used in the image display apparatus. As illustrated in these figures, in the conventional image display apparatus, an image that is displayed by an image display device is transmitted as an aerial image by a relay optical system that includes a positive lens, and the aerial image is projected into an observer's eyeball as an enlarged image by an ocular optical system formed from a concave reflecting mirror.
U.S. Pat. No. 4,669,810 discloses another type of convention image display apparatus. In this apparatus, as shown in FIG. 20, an image of a CRT is transmitted through a relay optical system to form an intermediate image, and the image is projected into an observer's eye by a combination of a reflection holographic element and a combiner having a hologram surface.
Japanese Patent Application Unexamined Publication (KOKAI) No. 62-214782 (1987) discloses another type of conventional image display apparatus. As shown in FIGS. 21(a) and 21 (b), the conventional image display apparatus is designed to enable an image of an image display device to be directly observed as an enlarged image through an ocular lens.
U.S. Pat. No. 4,026,641 discloses another type of conventional image display apparatus. In the conventional image display apparatus, as shown in FIG. 22, an image of an image display device is transferred to a curved object surface by an image transfer device, and the image transferred to the object surface is projected in the air by a toric reflector.
U.S. Reissued Pat. No. 27,356 discloses another type of conventional image display apparatus. As shown in FIG. 23, the apparatus is an ocular optical system designed to project an object surface onto an exit pupil by a semi-transparent concave mirror and a semitransparent plane mirror.
Other known image display apparatuses include those which are disclosed in U.S. Pat. Nos. 4,322,135 and 4,969,724, European Patent No. 0,583,116A2, and Japanese Patent Application Unexamined Publication (KOKAI) No. 7-333551 (1995).
In these conventional techniques, however, a reflecting surface and a transmitting surface, which constitute an optical system, are formed by using a spherical surface, a rotationally symmetric aspherical surface, a toric surface, an anamorphic surface, etc. Therefore, it has heretofore been impossible to favorably correct ray aberration and distortion at the same time.
If an image for observation is not favorably corrected for both aberration and distortion, the image is distorted as it is viewed by an observer. If the observation image is distorted such that images viewed with the user's left and right eyes are not in symmetry with each other, the two images cannot properly be fused into a single image. In the case of displaying a figure or the like, the displayed figure appears to be distorted, making it impossible to correctly recognize the shape of the displayed figure.